Alluvial Environments Research Articles

Late Quaternary Landscape Dynamics at the La Spezia Gulf (NW Italy): A Multi-Proxy Approach Reveals Environmental Variability within a Rocky Embayment

The Gulf of La Spezia (GLS) in Northwest Italy is a rocky embayment with low fluvial influence facing the Mediterranean Sea. Past landscape dynamics were investigated through a multi-proxy, facies-based analysis down to a core depth of 30 m. The integration of quantitative ostracod, foraminifera, and pollen analyses, supported by radiocarbon ages, proved to be a powerful tool to unravel the late Quaternary palaeoenvironmental evolution and its forcing factors. The complex interplay between relative sea-level (RSL), climatic changes, and geomorphological features of the embayment drove four main evolution phases. A barrier–lagoon system developed in response to the rising RSL of the Late Pleistocene (likely the Last Interglacial). The establishment of glacial conditions then promoted the development of an alluvial environment, with generalised erosion of the underlying succession and subsequent accumulation of fluvial strata. The Holocene transgression (dated ca. 9000 cal year BP) caused GLS inundation and the formation of a low-confined lagoon basin, which rapidly turned into a coastal bay from ca. 8000 cal year BP onwards. This latter environmental change occurred in response to the last Holocene stage of global sea-level acceleration, which submerged a morphological relief currently forming a drowned barrier-island complex in the embayment.

The Serbian Lake Pannon formations - their significance and interregional correlation

The problem of correlating Lake Pannon sediments across its basin has been the occupation of many geologists. At first, it was hampered by the prevalence of biostratigraphic, rather than lithostratigraphic correlation. The task became accomplishable when, thanks to seismic survey data, the strongly progradational character of Lake Pannon sedimentation had been understood. Thus, this paper aims to describe the formations from all parts of Lake Pannon and compare them to the ones described in Serbia. Material used includes published and unpublished data from all countries with Pannonian Basin System upper Miocene and lower Pliocene deposits, in the form of seismic, borehole and outcrop data. Even though the system is strongly asymmetric, both spatially and temporally, the formation synthesis framework should help better understanding among geologists operating within the basin. For the first time the informal formations are proposed for all Lake Pannon sediments in Serbia. The formations are linked to a progradational deltaic system within the following succession: basinal plain?turbidite?slope?delta front?delta plain?lacustrine and alluvial environments. The lithostratigraphic correlation has a huge potential in the context of industry. The main potential surely lies in petroleum geology, but it could be also very useful for exploration of geothermal energy, hydrogeology and construction materials.

Quantification of Surface Water-Groundwater Exchanges by GIS Coupled with Experimental Gauging in an Alluvial Environment

Surface water and groundwater are two interrelated components, where the influence of one automatically affects the quantity and quality of the other. These exchange flows are robustly influenced by some mechanisms such as permeability, lithological nature of the soil, landscape, in addition to the difference between the hydrometric height of the river and the piezometric level of groundwater. The study area of Bou Ahmed plain is vulnerable to intensive pumping mainly in the coastal fringe. The increase in water demand, due to demographic development, is accompanied by pressure on groundwater abstraction which causes significant drops of the groundwater level. The main objectives of this study are to develop Geographic Information System database and mathematical models to analyze spatial and temporal hydrogeological characteristics and hydrodynamic functioning of groundwater flow of the Bou Ahmed aquifer. The present work exhibits the characteristics of the river-groundwater exchanges in an alluvial plain. Therefore, we quantified the flows exchanged between a river and its groundwater using GIS tools along with measurements of parameters obtained by the differential gauging, which was carried out in the field, and hydrogeological boreholes data. These quantified flows, moreover, enabled us to eventually estimate the uncertainties related to the use of the GIS method. These results will also be used to support a set of groundwater simulations based on MODFLOW code in the Bou Ahmad aquifer. These models also associated with develop Geographic Information System will help to better plan, manage and control the groundwater resources of this aquifer.

Recent Vertebrate and Invertebrate Burrows in Lowland and Mountain Fluvial Environments (SE Poland)

Over geological time, fauna inhabiting alluvial environment developed numerous traces of life, called bioturbation structures. However, present literature rarely offers comprehensive and comparative analyses of recent bioturbation structures between different types of environments. In this paper, the distribution of recent animal life traces in two different fluvial environments (lowland and mountain watercourse) was examined. An analysis of a set of vertical cross-sections of river bank enabled to determine physical and environmental features of the burrows, as well as degree of bioturbation in individual sections. Most of the burrows were assigned to their tracemakers and compared between two studied reaches in relation to the geomorphic zones of a stream channel. A mesocosm was conducted using glass terraria filled with river bank sediment and specimens of Lumbricus terrestris. The experiment confirmed field observations on the ability of earthworms to migrate into deep sediment layers along plant roots. The impact of floods on fauna survival was assessed on the basis of observations of large floods in 2016, 2018 and 2019. The abundance, distribution and activity of fauna in the sediment are mainly controlled by the occurrence of high and low flows (droughts and floods), which was particularly visible in lowland river reach.

Modelling the trends of resistivity gradient in hydrogeological units: a case study of alluvial environment

Generally, longitudinal resistance is identified as one of the second order geo-electric indices used to assess aquifer vulnerability to contamination; mapping of geologic sequence for location of engineering sites and evaluation of subsurface for mineral explorations based on the varying electrical characteristics. Geo-resistivity of vertical electrical sounding (VES) was performed to examine the variations of resistivity gradient for surficial geological lithologies in the alluvial environments. Fourteen VES employing Schlumberger electrode configuration with a maximum current spacing AB of about 300 m were performed. The VES data were reduced to its first order geo-electric indices, representing geologic equivalents of the layers penetrated by current with the constraints of nearby logged boreholes. The first order geo-electric indices were converted to second order geo-electric indices, which include longitudinal conductance and resistance. The injected current at its maximum electrode separations penetrated four sedimentary units characterized by resistivity range of 95.2–3455.5 Ωm and mean value of 834.8 Ωm in the first layer. In the second layer, the gamut of resistivity was found to be from 8.8 to 3606.6 Ωm while its average value was 1327.8 Ωm. In the third and fourth layers, the mean values of resistivities were 657.2 and 1551.2 Ωm while their ranges were 72.5–1464.5 Ωm and 117.1–4893.0 Ωm respectively. Two sets of reflection coefficients estimated between layers 1 and 2 and 2 and 3 were determined and used to assess the trend of distribution of the geologic units, which indicated that the layers are alternating with argillites and arenites. Both the longitudinal conductance and resistance estimated indicate that the assessed alluvial environments are on the average, vulnerable to contaminations.

Engineering concept for reclamation on land use of abandoned alluvial tin mine as a solution to achieve sustainable post-mining

Hundreds of abandoned tin mines ponds in Bangka Belitung Province are not reclaimed properly during the post-mining activities. Reclamation in the former mining area ideally is a series of ongoing activities with planned post-mining activities. Planning for reclamation of abandoned tin mines or that has been re-mined by the community is a challenge to represent a concrete manifestation of sustainable management in the Indonesian mining sector to fulfill Government Regulation No. 78/2010 and the Minister of Energy and Mineral Resources Decree No. 1827/2018. This study purposes are to formulate the concept of the sustainable reclamation for the alluvial tin mines. Studies that have been conducted are to portrait topographic area by using drone, then followed by hydrology and hydrogeological mapping, infiltration and pumping test, shallow drilling and geohazard study. Air photo using drone can be used for topographic mapping. Ground water flows from the upper part area to central low land area which represents a river with low gradient. Three geohazards are identified such as flood, erosion and minor landslide. Based on the identified geohazards, the detail design of building ditch, sediment trap and reshaping landform are prepared carefully for the alluvial environment.

3D geological modeling and engineering properties of shallow superficial deposits: A case study in Beijing, China

Three-dimensional (3D) geological modeling has been developed in recent decades to overcome the complex nature of shallow superficial deposits in urban areas as a result of urbanized advancement. To meet the requirements of 3D geological models in the urban underground space construction in Beijing City, we constructed a coupled 3D model of geological sequences setting up to 50 m from the surface, and geotechnical properties of subsoils in Beijing for the first time via Creatar Xmodeling software. The modeling relies on 107 intersecting cross-sections and the horizons-to-solids approach aims to provide an insight into related modeling projects, especially in a complicated alluvial environment. In this paper, we described detailed modeling procedures, exhibited the outputs from the 3D model (combined geological units, cross-sections or fence diagrams, and properties presented in the form of horizontal distribution maps), and analyzed typical engineering properties of silty clay and clay units. The achieved 3D attributed model provides geologists an effective method to visualize the property characteristics of each layer and the potential connections between them, thereby to predict underground conditions and to reduce risks in geotechnical engineering processes such as urban underground constructions.

LATE PERMIAN PALEOENVIRONMENTAL CHANGES IN THE XINGCHENG AREA, LIAONING PROVINCE, CHINA: SEDIMENTARY SUCCESSION AND ROOT SYSTEMS

ABSTRACT During the late Permian, an alluvial environment prevailed along the northern margin of the North China Plate in the Xingcheng area, Liaoning Province, China. In this region, the middle–late Permian Shihhotse Formation and the late Permian Hamashan Formation record this transition from previous swamp-dominated settings. The two formations are mainly coarse-grained deposits and can be correlated with the Pingdingshan Sandstone, which developed as widespread alluvial facies in the central and southern parts of the North China Plate. Tectonic uplift around the plate due to Variscan orogenic activities probably contributed to the onset of alluvial prevalence, but the sedimentary characteristics in the Xingcheng area indicate that aridification was also an important driver. This is supported by the presence of root systems composed of rhizoliths in the Shihhotse Formation and Stigmaria fossils at the top of the alluvial conglomerate of the Hamashan Formation. Viewed at a larger scale, this basin-wide alluvial episode on the North China Plate was contemporaneous with the Wuchiapingian extreme aridity and followed a short humid interval. The overlying Changhsingian–Induan redbeds mark a paleoclimatic change to semiarid and arid again. The paleoenvironmental changes recorded in the upper Permian and Lower Triassic strata of the North China Plate, therefore, provide essential information for the understanding of late Paleozoic terrestrial evolution.

Probabilistic 3D modelling of shallow soil spatial variability using dynamic cone penetrometer results and a geostatistical method

ABSTRACT Since soils are natural and non-homogeneous materials, spatial variability of their properties has been recognised as one of the main sources of uncertainties affecting geotechnical analyses. However, soil testing is limited and the description and quantification of the resulting soil variability still remain largely subjective in practice. In this paper, anapproach is proposed to rationally evaluate the soil spatial variability using field data provided by a site investigation programme based on a lightweight dynamic cone penetrometer test. The first part deals with the boundary identification of statistically mechanical homogeneous soil units. The second part focuses on modelling spatial variability through 3D conditional random fields in the homogeneous soil units previously identified. This approach has been applied to and studied in a real site investigation carried out in an alluvial Mediterranean deltaic environment.

Drag coeffcient and hydraulic roughness generated by an aquatic vegetation patch in a semi-arid alluvial channel

Vegetation is one of the main factors that compose the alluvial dynamic of the semiarid environment. The determination of the hydraulic roughness generated by the plant in the alluvial environment is essential to understand the behavior of the forces that describe the flow, the processes associated with the loss of energy and the sediment transport rates. The objective of this study is the characterization of the flow resistance imposed by an aquatic vegetation patch in an intermittent stream, located in Pernambuco's hinterland, Brazil, in the emerged and submerged conditions, through the parameters of vegetal drag coefficient (CD) and vegetal drag stress (τveg) of the different intrinsic species of this alluvial environment. The sampling took place in the Cachoeira intermittent stream to monitor the channel geometry, vegetative parameters, and liquid and solid discharges. Based on the submergence ratio, the flux was divided into two distinct layers that allowed the understanding of hydro-sedimentological dynamics, the first one of direct contact with the vegetation and the second one the free flow zone. The average values of CD to the aquatic vegetation patch (composed by Senna obtusifolia, Ipomoea pes-caprae and Cynodon dactylon (L.) Pers.), according to the methodologies adopted in this study, were 10.97 and 3.73 m−1. The generated physical forces were in opposition to the flow and, consequently, in the condition of increase of the vegetal density, they promoted the higher values of vegetal drag stress, and retarded the flow velocity, reducing the transport rate of suspended sediment.

New 40Ar/39Ar, fission track and sedimentological data on a middle Miocene tuff occurring in the Vienna Basin: Implications for the north-western Central Paratethys region

Abstract The Kuchyňa tuff is found on the Eastern margin of the Vienna Basin and was formed by felsic volcanism. The Ar/Ar single grain sanidine method was applied and resulted in an age of 15.23±0.04 Ma, which can be interpreted as the age of the eruption. The obtained numerical age is in accordance with the subtropical climate inferred by the presence of fossil leaves that originated in an evergreen broadleaved forest. Furthermore, the described volcanism was connected with the syn-rift stage of the back-arc Pannonian Basin system. The sedimentological data from the underlying sandy mudstones indicate alluvial environment what confirms terrestrial conditions during deposition. Moreover, the tuff deposition probably occurred shortly before the Badenian transgression of the Central Paratethys Sea.

Subsidence mechanisms and sedimentation in alluvial sinkholes inferred from trenching and ground penetrating radar (GPR). Implications for subsidence and flooding hazard assessment

Sinkholes function as small sediment traps that may host valuable stratigraphic records for paleoenvironmental reconstructions and hazard assessments (e.g., subsidence, floods, hurricanes, tsunamis, tephra fall-out). The sedimentological features and completeness of such archives are influenced by the sedimentation and subsidence patterns and rates. However, karst depressions are frequently treated as static basins unaffected by settlement. This work illustrates the practicality of integrated studies combining trenching, numerical dating and shallow geophysical techniques (GPR) for characterizing the subsurface subsidence structure associated with sinkholes and reconstructing their deformational and sedimentary history. The approach is applied in two collapse sinkholes located in contrasting geomorphic settings (relict terrace and floodplain) related to deep-seated interstratal karstification of evaporites. The analysis of the sinkholes, particularly the trenching technique, provides practical information for assessing the associated subsidence hazard, including the presence of larger cavities at depth, the kinematic regime (episodic versus progressive), evidence of catastrophic displacement (fluidization structures) and the magnitude and timing of collapse events, especially the most recent one. The sinkhole located in the floodplain offers the opportunity of analyzing the possibilities and limitations of subsidence sinkholes as recorders of past floods in alluvial environments. This depression shows a largely incomplete record attributable to the high frequency of flood events compared to that of the collapse events, which create the accommodation space for sediment deposition. These limitations could be partially overcome by selecting old sinkholes situated in low terraces and/or affected by rapid subsidence.

Evidence for Late Alluvial Activity in Gale Crater, Mars

AbstractThe occurrence of past habitable conditions in Gale crater is generally associated with lacustrine and alluvial environments present >3 Ga ago, during the Hesperian Period on Mars. However, later‐occurring aqueous activity is consistent with superposition relations between some alluvial deposits and bounding materials on the crater walls, preservation of fine‐scale morphology on these alluvial deposits, and their superposing crater densities. The alluvial deposits include some not previously considered, and collectively lend confidence to the interpretation that local aqueous activity persisted in Gale crater into the Amazonian, or <2 Ga ago. Our conclusions are generally in accordance with late aqueous activity inferred from geochronology data (Martin et al., 2017, https://doi.org/10.1002/2017JE005445), in addition to late alluvial activity elsewhere. Interpreted late aqueous activity points to possible habitable settings in Gale later than previously recognized.

Detrital zircon U–Pb insights into the timing and provenance of the South Pyrenean Jaca basin

We investigate how sedimentary systems respond to tectonic uplift during the last stages of infilling of a foreland basin. Massive conglomeratic deposits characterize the last stage of basin fill in the Jaca basin, the western portion of the South Pyrenean foreland. We conduct detrital zircon U–Pb geochronology in a 3000 m thick Cenozoic section containing the transition from deltaic to alluvial environments with the aim of reconstructing changes of provenance in this key stage of evolution of the basin. The last conglomeratic deposits of the basin belong to the San Juan de la Peña alluvial fan, which has previously been assigned an early Oligocene age. Our results show the presence of earliest Miocene detrital zircons at the base of the fan. Zircon double-dating reveals identical U–Pb and (U–Th)/He ages, which is indicative of a volcanic origin for these zircons. We therefore conclude that this fan was deposited during early Miocene times. Our data establish a new chronological framework for deformation and sedimentation in the South Pyrenean foreland, with strong implications for the dynamics and palaeogeography governing the last episodes of basin fill. Our results support synchronous sedimentation in the Jaca piggy-back basin and in the autochthonous Ebro basin during the last stages of Pyrenean compression in the Miocene. This work emphasizes the tectonic controls on the configuration of the drainage basin and gives insights into the evolution of the drainage network in internally drained basins. Supplementary material: S1: Extended Zircon U-Pb LA-ICP-MS methodology. S2: Reduced detrital zircon U-Pb dataset for all analysis. S3: All age spectra. S4: zHe data. S5: Sample location for the analyzed samples are available at: https://doi.org/10.6084/m9.figshare.c.4507073

A geochemical and micro-textural comparison of basalt-hosted chalcedony from the Jurassic Drakensberg and Neoarchean Ventersdorp Supergroup (Vaal River alluvial gravels), South Africa

South African basalt-hosted chalcedonies provided an ideal case study, to (a) achieve information about the origin of mineral-forming fluids, the process of chalcedony formation and weathering alteration and (b) to verify if the geochemical and micro-textural characterization of chalcedony allowed multiple provenances to be distinguished. For the first time, Drakensberg chalcedonies from the Karoo Jurassic basalts and Windsorton chalcedonies from the alluvial environment of the Vaal River (traversing the Allanridge basaltic–andesitic lavas) were investigated by optical microscopy, laser ablation inductively coupled plasma mass spectroscopy and isotope ratio mass spectrometry (16O, 18O). The results showed that the compositional differences observed in Drakensberg and Windsorton chalcedonies could not be explained by changes in host rocks composition. The tholeiitic basaltic–andesitic lavas of the Allanridge Formation (Pniel Group, comparison term for Windsorton samples) and the tholeiitic basaltic–andesitic lavas of the Golden Gate Unit (Drakensberg Group, comparison term for Drakensberg specimens) proved to be very similar, also in terms of alteration degree. Conversely, both geochemical and oxygen isotope compositions clearly supported a relatively low temperature, hydrothermal origin for all the investigated chalcedonies, further indicating that differences mostly occurred during their formation (fluid circulation). Moreover, several characteristics suggested deposition in non-equilibrium conditions, although oxidizing conditions were most likely. Weathering processes were effective in the alluvial environment only, leading to a wider compositional heterogeneity of Windsorton chalcedonies with respect to Drakensberg samples. Lastly, a correlation between color and chromophores (Fe and Mn) amounts was lacking while a correlation between the geochemical composition and the texture was clearly observed in several specimens.

Revisiting the age and palaeoenvironments of the Upper Jurassic–Lower Cretaceous? dinosaur-bearing sedimentary record of eastern Spain: implications for Iberian palaeogeography

An integrated stratigraphic, palaeontological, palaeoenvironmental and palaeogeographical study of the traditionally considered Upper Jurassic–Lower Cretaceous dinosaur-bearing sedimentary record (DSR) of eastern Spain is accomplished for the first time. Several areas where dinosaur fossils are abundant (western Maestrazgo and South-Iberian basins) have been studied in detail. In all the areas, the DSR comprises a carbonate-dominated lower part (CLP), and an essentially siliciclastic upper part (SUP). Deposition occurred in a shallow-very shallow marine carbonate platform, laterally connected towards the N and W to coastal and alluvial environments. The overall upwards evolution is regressive with a transgresive episode at the uppermost part. The DSR includes deposits previously assigned, depending on the studied area, from the Kimmeridgian to the Barremian (locally even to the Aptian–Albian). However, ages obtained in this work from larger benthic foraminifera (LBF), demonstrate a Kimmeridgian–Tithonian age (locally Kimmeridgian-Early Berriasian?) for the DSR. These findings have important implications regarding the age of dinosaur fossils of these deposits, traditionally assigned to the Jurassic-Cretaceous transition, or even to the Early Cretaceous, erroneously, and have necessitated a deep litho- and chronostratigraphic revision of the units previously established in the studied areas: new data indicate that the DSR is correlatable with deposits of the Villar del Arzobispo Fm and that the usage of the Aldea de Cortes and El Collado Fms, traditionally assigned to the Early Cretaceous, should be avoided. New data also reveal that the DSR should be correlated with other Kimmeridgian–Tithonian dinosaur-bearing deposits of Iberia, such as those of the Cameros Basin, Asturias and Portugal, and have encouraged a revision of the Iberian palaeogeography at that time. In fact, ages obtained from LBF agree with data provided by the systematics of dinosaurs, since dinosaur faunas of eastern Spain are similar to those of the other Late Jurassic Iberian areas, especially to those of the Lusitanian Basin.

The pre‐79 CE alluvial environment south of Pompeii's city walls

AbstractThe environment in the suburban area of Pompeii between Porta Marina and Porta Stabia at the time of the 79 CE volcanic event was investigated by means of a series of core drillings. These revealed the presence of a ubiquitous dark brown layer (referred to as “Phase 2 interval”) immediately below the 79 CE eruption deposits. Its stratigraphic interpretation relied on 14C dates, diatom analysis and micromorphology, and on comparisons and correlations with the available stratigraphic logs and excavation data. Phase 2 interval represents the sedimentary accretion that took place from ca. 900–750 cal a BCE to 79 CE. Its formation is the result of periodic flooding by the Sarno river in this distal reach of its floodplain (i.e., backswamps). The resulting picture is that of a patchy environment with weakly developed alluvial soil juxtaposed to areas with stagnating waters. No indication of artificial or natural watercourses derived from the Sarno river, of navigable lagoons, or of harbor basins was found.

Campanian-Maastrichtian and Eocene stratigraphic architecture, facies analysis, and paleoenvironmental evolution of the northern Magallanes Basin (Chilean Patagonia)

The stratigraphic architecture and environmental evolution of the Cerro Guido-Rio de las Chinas Valley Complex contains the upper Cretaceous to Eocene record of the Magallanes/Austral Basin, located in southernmost Chile. This retroarc foreland basin contains a significant record of early Cretaceous to Miocene biogeographic and environmental changes that occurred in high paleolatitudes after the break-up of Gondwana. Using 37 stratigraphic sections, we generated a 1390 m thick stratigraphic column, which was used to developed sedimentary facies and an analysis of sequence stratigraphy. Results show, in general, aggradation composed of six third-order depositional sequences each delimited by an erosive surface. A lowstand systems tract developed above each surface, represented by alluvial environments similar to braided fluvial systems, and coastal plain environments, with the development of meandering fluvial and lacustrine systems. Above the lowstand tract, the transgressive systems tract is composed of a shallow marine environment with foreshore, shoreface, and offshore systems. A highstand systems tract was observed in one sequence, and is represented by shallow marine facies. The extensive outcrop in the studied area contains a diverse and voluminous fossil record, containing fossilized invertebrates (bivalves, gastropods, bryozoans, and brachiopods), vertebrates (hadrosaurs, ornithischians, sauropods, theropods, birds, mammals, frogs, turtles, plesiosaurs, mosasaurs, and sharks), and plants (wood, leaves impressions, flowers, pollen, and spores). Thus, this study broadens the understanding of the South American-Antarctic connection during the late Cretaceous-Paleogene, through a diverse fossil record and study of the paleoenvironmental evolution of this high-latitude basin.

Contribution of gravity and Bristow methods for Karez (aqueduct) detection

Karez (aqueduct or Qanat) is a water supply system used for hundreds of years in a number of different regions across the world. Due to the extension of constructions close to old networks of Karezes, the detection of path and depth of the Karez is very important. The Karez network understudy is embedded in the alluvial environment in which mostly clay material surrounds the channel. Further, the ceiling of Karez in the survey area has a frequent depth of 1–2 m. to the surface. Initially, to identify the path and boundaries of the Karez's network, three gravity profiles were carried out. After processing, a derived residual map of Bouguer anomaly revealed the path of the Karez. Applying the relatively recent method of Euler deconvolution in a radial coordinate system yields the depth of 1.4 m. from the surface to the center of Karez. In addition to this, a successful technique in determining the depth of underground cavities, the Bristow method was carried out. Actually, soft and low resistivity soil around the Karez channel was the biggest challenge in the interpretation of conducted pole-dipole electrical arrays. Therefore, a forward modeling based on the finite difference method was used in order to model pole-dipole resistivity array for a round-shaped object with two various diameters in several depths which looks the same as a Karez. The results of the models were used in Bristow graphical interpretation. The average of measurement error for depth estimation obtained by this method is about 7%. Further, the interpreted field data obviously revealed the depth of 1.8 m. from the center to the top of Karez, which agrees well with gravity results.

Composition and diagenesis of Neoproterozoic Guaritas Group sandstones in the Minas do Camaquã fault zone, Camaquã Basin

Geological structures influence the formation and evolution of sedimentary rocks. The Minas do Camaquã fault zone is a primary structure of the Camaquã Basin, controlling the uplift of the ore-bearing units of the Santa Bárbara Group. To the south of the Camaquã River, the fault zone deforms alluvial and eolian sequences attributed either to the Santa Bárbara or Guaritas groups. In this study, a facies and petrographic composition and diagenetic analysis are presented to understand the evolution of the fault zone. Facies analysis was accomplished using high-resolution orthophoto mosaics and field surveys. Seven sedimentary facies were defined, grouped into three facies associations. Facies associations correspond to a succession of climate-influenced depositional environments. The transition from humid to dry conditions occurs from a fluvial (facies association 1) to eolian environments (facies association 2). These deposits are overlaid at the top by a high energy environment deposits characterized by amalgamated gravelly and sandy bodies, corresponding to an alluvial environment (facies association 3). Despite a small compositional variation, sandstones present a continental block provenance which may be related to mixed anorogenic or orogenic provenance. Diagenetic features are similar in the three facies associations, suggesting the same burial history for the sedimentary deposits separated by the fault zone. The Minas do Camaquã fault zone in the study area is an intraformational structure, as analyzed sequences are attributed to the Guaritas Group, implying a relatively high degree of deformation late after its deposition.